The worms are part of a group of species called hammerhead flatworms, which originated in tropical Asia.

The worms have gone largely unnoticed for close to two decades, according to scientists.

When an amateur naturalist kept digging up strange, flat-headed worms in his garden outside of Paris, he knew something was amiss.

Pierre Gros, a naturalist, emailed a picture to Jean-Lou Justine, a zoologist at the Muséum National d’Histoire Naturelle in Paris. Justine thought it was a fake, but Gros persisted and kept sending in more images of the huge worms.

“The man is bringing back worms from his travels, and he pretends he finds them in his garden!” Justine told The Washington Post of his thoughts upon seeing the images.

But the two were on to an unsettling discovery: These worms, native to tropical Asia, were invading France – and nobody had noticed.

Gros and Justine teamed up on a new paper, published in the journal PeerJ, that documents occurrences of these native worms in France. As it turns out, these quiet invaders have been occupying French territory for almost two decades.

They have aso been found everywhere from England to French colonies in the Caribbean.

The worms are part of a group of predatory species known as hammerhead flatworms, which grow to over a foot long. They prey on earthworms in the soil and produce a potent cocktail of chemicals to immobilize their prey.

Justine and Gros identified three species – which mostly turned up in France’s warmer south – that they now believe have been in the country since at least 1999.

Scientists like Justine are worried about the effects these invasive species can have on local critters. Earthworms are crucial for aerating soil and ensuring agriculturally productive land. When new predators are introduced, like the giant flatworms, it can throw the whole, delicate ecosystem out of whack.

One of the most surprising parts of the discovery is that these worms escaped notice until now.

“It is France! It is supposed to be a developed country. We have a lot of scientists; we have universities everywhere,” Justine told The Post.